We propose to genetically and molecularly characterize the nucleolus organizer regions of the X and Y chromosomes and X-No flanking sequences in D. melanogaster. By selectively changing the rDNA multiplicity of the X-NO in four distinct ways--compensation, independent rDNA polytenization, abo reversion and magnification--we will determine if certain rDNA repeat types are preferentially replicated or undergo unequal exchange during these events. It is intended to correlate the in vivo transcriptional inactivity of interrupted repeats with their generalized DNase insensitivity suggesting an altered chromatin environment to that of the uninterrupted repeats. Five nucleolar organizer mutants of the Y chromosome are analyzed by gene numbers, additivity tests, phenotypes, kinds of repeats and DNase sensitivity and thus profile Y-NO gene activity with development. We hope to identify and genetically localize non-rDNA sequences of Xh from two constructed DNA libraries of D. melanogaster.